Data on modern computer disks is stored along circular tracks that are written and read by servo controlled, magnetic read/write heads. In order to allow for precise guidance of the read/write heads, disks are further coded with guide tracks. Typically, such guide tracks encode clock signals at defined intervals, and are placed on the disk surface during the manufacturing process. In order to allow for uniform guidance, clock signals are evenly spaced in the clock track. Even spacing can be assured by producing a clock signal at controlled, even, intervals. However, unless the period of rotation is an integer multiple of the nominal interval between two clock signals, the distance between the last clock signal and the first clock signal on the track will not be equal. Such a clock track is said not to be “closed”, and causes jitter in any clock recovered from the track.
To ensure precise tracking, each time a clock track is written on a disk, clock track closure is ensured. To accomplish this, a clock track is currently re-written as many times as necessary to guarantee satisfactory clock track closure. This may be time-consuming and increases manufacturing throughput time.
Additionally, even once clock track closure is achieved, a clock track typically continues to include written-in jitter. Written-in jitter results in phase-in error of propagated servo-patterns. This in turn affects the quality (linearity) of the position error signal.
Written-in jitter is particularly acute, when media level servo track writers (MLSTW) are used in the production of disks. Currently, a clock track is written on a reference disk that is used each time a new set of disks is loaded on the MLSTW. When the reference disk is loaded on MLSTW, the offset of the disk will be different from the offset when the disk was written, and therefore the center of the written clock track on the disks will change. This will result in repeatable jitter of the read-back clock signal on the produced disks. This repeatable jitter has to be compensated to ensure precise propagation of the servo patterns on the blank disks.
Accordingly, there is a need for a clock tracking circuit that may more effectively compensate for various types of clock jitter, including jitter attributable to incorrect clock track closure, and written-in jitter.